In connection with the progress toward higher accuracy in satellites and telescopes, higher integration in semiconductor devices, higher precision in aspherical lenses, etc., there is a growing need for components used in space and astronomical devices, manufacturing apparatuses and measuring apparatuses for the semiconductor devices to ensure high shape (dimensional) accuracy and high long-term stability of shape accuracy. Moreover, the components for the devices and the apparatuses has become very complicated and fine in a shape in recent years. As a result, fine grinding is also being increasingly required.
Meanwhile, the above ultra-precision apparatuses require a mirror for positioning or a reference mirror for reflection of laser or extreme ultraviolet rays, an aspherical mirror for light focus and the like. Therefore, the above precision mirrors are being increasingly required to have a significantly smooth surface (a surface having small surface roughness). Moreover, as a material for the mirrors, high quality materials from which super smooth surfaces can be obtained have been required. As a high-quality material from which the super smooth surface can be obtained, the material disclosed in the following Patent Document 1 is known.
Specifically, the Patent Document 1 discloses “a cordierite-based sintered body which comprises cordierite as primary components, and the cordierite-based sintered body contains one or more selected from the group consisting of La, Ce, Sm, Gd, Dy, Er, Yb and Y in an amount of 1 to 8 mass % in terms of oxide, without any crystal phase other than a cordierite crystal phase, wherein a mass ratio between respective ones of the primary components satisfies the following relations: 3.85≤SiO2/MgO≤4.60, and 2.50≤Al2O3/MgO≤2.70, and the cordierite-based sintered body after being subjected to precision polishing has a precisely polished surface with an average surface roughness (Ra) of 1 nm or less,” The invention described in the Patent Document 1 makes it possible to drastically improve a precision polishing characteristic of a cordierite-based sintered body which has low thermal expansibility, high stiffness (high elastic modulus) and high dimensional long-term stability.
However, even with the cordierite-based sintered body of the Patent Document 1, complex and fine processability does not have been to be possible to be achieved. Specifically, it has not been obtained so far any low thermal expansion material which has both of excellent precision polishing characteristic (high accuracy mirror processability) and complex and fine processability.